Some types of mass spectrometer require charged particles (typically ions) to be injected as a packet into a mass analyzer at elevated kinetic energies such as keV to facilitate high mass resolution operation. Examples include electrostatic trap (EST) and time of flight (TOF) mass spectrometers. In addition, typically these types of mass spectrometers, in common with some others, utilise charged particle detectors which convert incoming ions to electrons by physical processes which depend to some extent upon the velocity of the incoming particles. For efficient conversion, ions should impinge upon a detector surface with sufficiently high velocity. Such detectors can suffer from inefficient conversion characteristics for ions of high mass which, whilst having the same kinetic energy as lower mass ions, have a lower velocity. It is known to be advantageous to accelerate the ions to still higher kinetic energies to overcome this problem. However, where such acceleration occurs after mass analysis, it can have a detrimental effect on TOF mass resolution due to the process of post acceleration introducing temporal aberrations.
In EST and TOF mass spectrometers, the source of ionised particles may be a continuous source, such as electrospray ionisation, operating at atmospheric pressure. Other forms of atmospheric pressure ionisers are well known. Unless the ioniser produces pulses of ions within vacuum, it is normal to have distinctly separate ionisers and pulsed ion sources, the latter producing a packet of ions within vacuum for injection into the mass analyzer, the ions having been created initially by the ioniser. Where the ioniser operates outside the instrument vacuum system, it is advantageous that it operate at or near ground electrical potential to ease design constraints and for safety reasons. Ions produced must then therefore be increased in kinetic energy before injection into the mass analyzer. This has been accomplished in various ways.
In one way, an insulating beam transport element such as a glass capillary is located between the region at atmospheric pressure and an intermediate vacuum region. This type of arrangement is described, for example, in U.S. Pat. No. 5,965,883. The two ends of the capillary are metalized and electrical potentials applied so that the electrospray needle of the ionizer (as used in this example) is able to be maintained at ground potential. In this case a potential at −4500V was applied to the end at or nearest the atmospheric pressure region, and +160V to the end within the intermediate vacuum. The potential difference has a polarity so as to raise ions of a desired polarity to elevated potential relative to a mass analyzer. As such, the electric field within the capillary opposes the flow of ions along the capillary, but the capillary is made sufficiently long and the gas flow within the capillary is made sufficiently high to drive ions along the capillary channel overcoming the relatively weak opposing electric field. A disadvantage of this arrangement is the tendency to electrical breakdown of the gas due to the high potentials applied, and accordingly it has been found impractical to apply more than 4-5kV across the capillary. The result of this is that ions are only able to be raised to relatively low electrical potentials using this type of apparatus.
In another way, as described for example in U.S. Pat. No. 6,057,544, a drift zone consisting of a conductive tube is incorporated into the ion path between the ion source and the analyzer and a voltage is applied to the drift zone, switched between low potential and an accelerating potential of several kV in a shorter time than the time taken for the ions to traverse the tube. This method is unsuitable for raising all ions emitted from a continuous ion source to elevated potential.
In another way, the ions are moved through the instrument to the pulsed ion source, where they are trapped and cooled before injection into the mass analyzer. During the process of injection the electrical potential of a portion of the ion flight path immediately downstream of the pulsed ion source is changed, lifting the ions to an elevated potential as they fly through that portion of the flight path on route from the pulsed ion source to the mass analyzer. An example of such a system is described in U.S. Pat. No. 7,425,699 in relation to FIGS. 6 and 7, in which there is a curved trap and where an EST mass analyzer is used. Here, the ions are stored within the curved trap which forms the pulsed ion source, cooled by collisions with gas at 0.1-1 mTorr and then voltage pulses are applied to electrodes of the curved trap to eject the trapped ions. The curved trap may float at the accelerating potential—but then the ioniser must also be floated, so alternatively a liner is provided between the exit of the curved trap and lenses which precede the EST. Ions enter the liner on their flight path and a voltage pulse is applied to the liner to cause the liner to act as an energy lift. The potential of that portion of the flight path is raised whilst the ions are within the liner, and they emerge to be accelerated towards the mass analyzer which is maintained near ground potential. The curved trap may then also be maintained at or near ground potential, along with the ioniser. This arrangement is disadvantageous in that the liner is a finite length and this must be of sufficient length to contain ions of all the desired mass range as they fly from the pulsed source through the liner whilst the liner is raised to high potential, and this places a restriction on the minimum distance between the pulsed source and the mass analyzer. Ideally the distance between the pulsed source and the mass analyzer is kept as short as possible to reduce the time of flight separation of the packet of ions as they proceed from the pulsed ion source to the mass analyzer as otherwise it may limit the mass resolving power of the mass analyzer.
In a further way, ions are raised to the desired potential within the pulsed ion source at the time of the ejection of the ions. In this approach, the pulsed ion source is rapidly raised to high potential in order to extract the ions. However, pulsing electrodes from near ground potential to several kV to eject ions from the pulsed ion source is difficult because of the short timescale over which the pulse must take place, which may be of the order of ns, and because of the accuracy of the final voltage that must be achieved. The voltage applied must be free from ringing at the hundreds of mV level, or better. This is even more important when radio frequency (RF) pulsed sources are used where the RF and DC potentials are coupled and the RF potential alone must be switched off, complicating the electronic control. The accuracy of the final potential and any ringing of that potential during pulsing can have detrimental effects on the mass accuracy and the mass resolution achievable by the mass analyzer because of the effects they have upon the ion packet. Furthermore, where the accelerating potential is applied across the trap at the time of ejection, a large potential gradient exists within the trap. The spatial distribution of ions within the trap is finite and ions at different positions within the trap experience a different potential and are undesirably accelerated to different energies.
Mass analyzers that utilise packets of ions, such as EST and TOF analyzers, are ideally suited to use ionisers that produce pulsed beams, such as MALDI. However in order for them to utilise continuous beams of ions from sources such as electrospray ionisers, for example, the continuous beam must be sampled in some way. Pulses of ions have been extracted from the continuous beam in some prior art methods. However it is desirable for efficiency reasons to utilise as much of the continuous beam as possible, preferably all the continuous beam, so that desired detection limits may be achieved with the minimum of sample consumption. To accomplish high efficiency utilisation of a continuous ion beam, the beam may be accumulated in a store and accumulated ions formed into a packet for injection into the mass analyzer. Usually the ion beam is cooled by collisions with gas before injection to reduce the energy spread of the ions enabling high mass resolution to be achieved by the mass analyzer, and this may be accomplished in the accumulator, in the pulsed ion source, or both. For highest efficiency it is preferable that the accumulator and pulsed ion source can accommodate the entire output of the ioniser, but this has not yet been achieved for electrospray ionisers.
For some EST analysers, the duty cycle is dominated by the time to obtain high resolution data from the analyser. However high resolution may be obtained from some TOF analysers, for example, such as multi-reflection TOF analysers, in much shorter time periods, the time period being of the order of the time required to lift ions to the required energy for injection, and for these analysers it is especially valuable to be able to lift ions to the desired energy without disrupting the flow of ions through the instrument.
In view of the above, the present invention has been made.